I don't use a Mac for minetest.
To know where your mods must be placed, it might be helpfull if you can describe the map structure of your minetest game.
For example, on my Linux there are (dependend on the way they were installed) two structures possible (at least that I know off):
/usr/share/minetest/games/minetest_game/mods/NEONMOD
and
/home/TOPYWO/celeron55-minetest-8c3ffa3/mods/minetest/NEONMOD
I would think the Mac has a structure that looks similar to one of those.
Btw, I think Neon blocks are to new for the 0.4dev-20120122. You probably need to rewrite mods to make them work on your computer, or download the older versions or older mods.
Best thing for you would be to install the latest 0.4.3 for Mac. Pheonixfire and Omnistudent are trying to figure out how at the moment:
http://minetest.net/forum/viewtopic.php?id=3627